Abstract. Cirrus clouds have a net warming effect on the atmosphere and cover about
30% of the Earth's area. Aerosol particles initiate ice formation in the
upper troposphere through modes of action that include homogeneous freezing
of solution droplets, heterogeneous nucleation on solid particles immersed
in a solution, and deposition nucleation of vapor onto solid particles.
Here, we examine the possible change in ice number concentration from
anthropogenic soot originating from surface sources of fossil fuel and
biomass burning, from anthropogenic sulfate aerosols, and from aircraft that
deposit their aerosols directly in the upper troposphere. We use a version
of the aerosol model that predicts sulfate number and mass concentrations in
3-modes and includes the formation of sulfate aerosol through homogeneous
binary nucleation as well as a version that only predicts sulfate mass. The
3-mode version best represents the Aitken aerosol nuclei number
concentrations in the upper troposphere which dominated ice crystal residues
in the upper troposphere. Fossil fuel and biomass burning soot aerosols with
this version exert a radiative forcing of −0.3 to −0.4 Wm−2 while
anthropogenic sulfate aerosols and aircraft aerosols exert a forcing of
−0.01 to 0.04 Wm−2 and −0.16 to −0.12 Wm−2, respectively, where
the range represents the forcing from two parameterizations for ice
nucleation. The sign of the forcing in the mass-only version of the model
depends on which ice nucleation parameterization is used and can be either
positive or negative. The magnitude of the forcing in cirrus clouds can be
comparable to the forcing exerted by anthropogenic aerosols on warm clouds,
but this forcing has not been included in past assessments of the total
anthropogenic radiative forcing of climate.